Scientists at a California company reported yesterday that they had created the first mature cloned human embryos from single skin cells taken from adults, a significant advance toward the goal of growing personalized stem cells for patients suffering from various diseases.

Creation of the embryos -- grown from cells taken from the company's chief executive and one of its investors -- also offered sobering evidence that few, if any, technical barriers may remain to the creation of cloned babies. That reality could prompt renewed controversy on Capitol Hill, where the debate over human cloning has died down of late.

Five of the new embryos grew in laboratory dishes to the stage that fertility doctors consider ready for transfer to a woman's womb: a degree of development that clones of adult humans have never achieved before.

No one knows whether those embryos were healthy enough to grow into babies. But the study leader, who is also the medical director of a fertility clinic, said they looked robust, even as he emphasized that he has no interest in cloning people.

"It's unethical and it's illegal, and we hope no one else does it either," said Samuel H. Wood, chief executive of Stemagen in La Jolla, whose skin cells were cloned and who led the study with Andrew J. French, the firm's chief scientific officer.

The closely held company hopes to make embryos that are clones, or genetic twins, of patients, then harvest stem cells from those embryos and grow them into replacement tissues. When transplanted into patients, the tissues would not be rejected because the immune system would see them as "self."

"All our efforts are being directed toward personalized medicine and diseases," said Wood, adding that the scientists did not try to extract stem cells from the first embryos they made because they were focused on proving they could make the clones.

Other stem cell scientists expressed optimism but said they want to see the work repeated and more details presented.

"I'd really like to believe it, but I'm not sold yet," said Robert Lanza of Advanced Cell Technology (ACT) in Worcester, Mass. He said the report did not show the results of molecular tests that scientists typically do to prove that the cloning process was complete. He and George Daley, a stem cell scientist at Children's Hospital Boston, said the embryos look only marginally healthy in photos.

The work is the latest evidence, however, that the field is recovering from the scientific and public relations debacle of 2005, when similar claims by South Korean scientists proved to have been fabricated.

Nevertheless, opponents of research on human embryos lashed out at the approach.

"This study seems to confirm that human cloning . . . is technically possible," said Richard Doerflinger of the U.S. Conference of Catholic Bishops. "It does not show that a viable or normal embryonic stem cell line can be derived this way, or that any such cell has 'therapeutic' value. It does not answer the ethical or social questions about the mass-production of developing human lives in order to destroy them. . . . It only tells us that these questions are more urgent than ever."